FIG. 1 illustrates a typical optical encoding system. A code disc 20 is mounted to an encoder shaft, the angular position of which is to be monitored. The disc is generally glass and has a series of concentric annular code tracks inscribed thereon. Each track comprises alternate transparent and opaque segments defining equal parts around the shaft 22. The number of code cycles per track may vary from one cycle on the coarsest track 23 to several thousand cycles on the outer fine track 25. The actual angular position of the code disc can be determined from the instantaneous binary states of several code tracks.
In FIG. 1, the optical code reading system is shown for the fine track, it being understood that each track is similarly read. The track is illuminated by a light emitting diode (LED) 26. The thus illuminated transparent and opaque segments of the track are viewed by photodetectors 28 through precision optical slits 30. An instantaneous photodetector output is dependent on whether transparent or opaque segments are aligned with the photodetectors and their associated slits. The detectors associated with other than the finest track provide square wave outputs as the disc rotates, and together the outputs from several tracks represent a binary code.
The segments on the fine track are so closely spaced that they form a diffraction grating which provides for high fidelity sinusoidal photodetector outputs as the code disc rotates. The photodetector circuit providing the sinusoidal output is shown. in FIG. 2. Selected signals are shown plotted against angular position in FIG. 3.
To provide a first sinusoial output, designated the sine signal, two groups of slits 32 and 34 are precisely aligned with respect to the fine track 25 such that each is spaced a number of cycles plus 180 cycle degrees from the other. The detectors associated with the slits 32 and 34 are phototransistors 36 and 38 connected in a push-pull configuration. As shown, the slits 32 are aligned with transparent segments of the fine track 25; thus the transistor 36 conducts. On the other hand, the slits 34 are 180.degree. out of phase with respect to slits 32 and are aligned with opaque segments so that the transistor 38 does not conduct. As the code disc rotates, the two transistors 36 and 38 are illuminated alternately to provide outputs as shown in FIGS. 3a and 3b. The resultant output on line 40 is a sinusoid as shown in FIG. 3c.
To provide a cosine signal, two groups of slits 42 and 44 are positioned a number of cycles plus 90.degree. from respective slit groups 32 and 34. As a result, cosine detector transistors 46 and 48 are illuminated to provide the combined output on line 50 illustrated in FIG. 3f. It can be seen that the cosine signal on line 3f is advanced 90 cycle degrees from the sine signal of FIG. 3c.
The sine and cosine signals on lines 40 and 50 are processed to provide a position indication of a high resolution. That resolution would not be possible with a simply binary readout of the fine track 25. To that end, Sidney Wingate has shown in U.S. Pat. Nos. 3,310,798 and 3,312,838 that two square waves of the same frequency but out of phase can be logically combined, as in an exclusive-OR gate, to provide a new square wave of twice that frequency. If that signal having twice the frequency is then logically combined with a similar but out of phase signal, a signal having four times the frequency of the original signals can be provided. The multiple phase shifting necessary in such a method is provided by summing weighted sine and cosine signals from lines 40 and 50. The resultant phase shifted sinusoids are then converted to square waves for the logical combination noted above.
It is extremely important that the sinusoidal outputs on lines 40 and 50 have a precise phase relationship, equal amplitudes and no DC bias. If the phase relationship for which the signal weighting and summing electronics have been designed is not maintained, the phase shifted outputs from the electronics are distorted. On the other hand, the fine track may comprise black and white segments of no more than 10 microns (400 microinches) in length. With spacing of the code disc and detector optics necessary for free rotation of the disc, any minute misalignment of the mechanical components may result in a large distortion of the sine and cosine signals as to amplitude, DC offset and relative phase.
In the past, alignment of the photodetector and slit optics with respect to the LED and disc has been made by a highly skilled technician. The technician observes the output from the fine track on an oscilloscope as the code disc rotates. He switches the oscilloscope inputs for both sinusoidal signal displays and a Lissajous pattern display, an indication of the phase relationship of the signals. The technician must tap the detector and slit optics lightly in an effort to move them to a position which provides the desired oscilloscope displays. After moving the optics, the technician must adjust the oscilloscope gain and DC offset for each of the sinusoidal signals and then observe the Lissajous pattern to ascertain whether the movement of the detector and slit optics resulted in the desired phase relationship. In aligning optics with respect to discs having very fine code tracks, this procedure is extremely tedious and requires great skill in adjusting and reading the oscilloscope and in properly tapping the optics.
A primary object of this invention is to provide means for reducing the skill and time required to align code discs with associated code reading optics.